The present invention relates generally to marine seismic exploration. More specifically, the application pertains to streamer cables used in connection with marine seismic exploration. More particularly still, the invention relates to an in-line apparatus for retrieving a marine tool such as a seismic streamer cable, and a method for retrieving a cable using such an in-line apparatus.
In the exploration of hydrocarbons offshore, seismic surveys are often employed. During seismic exploration, a plurality of airguns or other acoustic transmitters are actuated in a survey area. The airguns are typically towed behind a boat offshore. The airguns are fired to create a series of underwater acoustic pulses that generate seismic waves. The waves travel downward through the water, down to the earth""s ocean bottom surface, and further downward through the various subsurface strata. Seismic waves reflect from the subsurface strata where they are then received by receivers placed in the water body. The receivers, known as xe2x80x9chydrophones,xe2x80x9d convert the physical seismic waves into electrical signals which are sent back to the ship for later processing. Data received from the receivers is processed in order to create a mapping of the underground formation.
Receivers such as hydrophones are carried through the water within cables referred to as xe2x80x9cstreamers.xe2x80x9d Typically, a seismic cable may be 3000 to 6000 meters long, or more. Marine seismic operations are conducted using one or more boats which tow the streamers through the water. The streamers incorporate the receivers at various intervals to define an array.
The streamers are typically designed with inherent buoyancy. The purpose is to maintain the receivers at a co-planar level within the water body. A separate cable-leveling device known as a xe2x80x9cbirdxe2x80x9d may also be employed to assist in maintaining the cables at a uniform depth. In this respect, it is desirable to maintain the cables at a uniform designated level within the water while acoustic pulses are being received.
In order to maintain positive buoyancy in a cable, a jacket in the cable is filled with a lightweight fluid such as a high-paraffin oil. The fluid will endow the cable with a buoyant characteristic. The fluid, or buoyant medium, provides the cable with a designated weight comparable to that of the surrounding water, i.e., less than a specific gravity of one. It also assists in maintaining the cable""s stable shape. Most importantly, the medium assists the cable at maintaining a substantially co-planar depth within the water. Additional background information concerning cable buoyancy is taught in U.S. Pat. No. 5,404,339 issued to Cole, Jr. on Apr. 4, 1995, which is incorporated in its entirety herein by reference.
Recently, various methods for maintaining a neutral buoyancy within a streamer cable have been developed. Examples include:
U.S. Pat. No. 6,019,652 entitled xe2x80x9cBuoyancy Adjustment.xe2x80x9d This patent was issued Feb. 1, 2000, to Nielsen, et al.
U.S. Pat. No. 6,142,092 entitled xe2x80x9cDepth Control Device.xe2x80x9d This patent was issued Nov. 7, 2000 to Coupland.
U.S. Pat. No. 6,188,646 entitled xe2x80x9cHydrophone Carrier.xe2x80x9d This patent was issued Feb. 13, 2001 to Luscombe, et al.
U.S. Pat. No. 6,239,363 entitled xe2x80x9cVariable Buoyancy Cable.xe2x80x9d This patent was issued to Wooters on May 29, 2001.
It is not uncommon for a marine cable streamer, such as the streamers described in the above patents, to lose buoyancy. Reasons for buoyancy loss include a severance of the cable, a disconnect of the cable from the boat, punctures within the cable, or other accidental causes. Loss of buoyancy of the cable oftentimes means that the cable and associated sensors and other seismic tools will be lost in the ocean, or at least rendered ineffective for seismic operations.
In order to recover lost cables and seismic sensors, it is desirable to incorporate a retriever mechanism into a seismic cable streamer or other marine tool. Cable retrievers today typically consist of modules that have the facilities and ability to inflate a bladder with gas in order to raise the streamer to the surface in the event of a loss of buoyancy within the cable streamer itself. At the present time, all such retrievers are independent modules which are mounted onto the cable using clamps. An example of a seismic cable recovery device is the Concord Technologies SRD-500(trademark), shown in FIG. 1. This is an automatic streamer recovery device 10 that aids in the recovery of seismic streamers 20 which have become severed from the towing vessel or have otherwise lost their buoyancy. The SRD-500 model includes quick-release mounting rings 14.
Seismic streamers are typically stored on a powered reel stack that is mounted on a seismic vessel. Each streamer is fed from the reel by hydraulic engines which rotate the reel in order to unspool the streamer. However, before a streamer cable can be deployed into the water, the deployment must be repeatedly interrupted in order to attach retrievers onto cable sections. Likewise, a cable cannot be spooled or otherwise retrieved onto the vessel without removing the retriever assemblies as they are recovered on deck. Thus, a significant amount of production time is lost in the attachment of the retriever modules during deployment of the seismic cables. Likewise, additional production time is lost by the detachment of the retriever modules when the seismic cables are retrieved.
The attachable/detachable structure 14 for retriever modules 10 and related tools, such as birds, has disadvantages. Streamers 12 can be up to six thousand meters in length and can require more than twenty retriever modules 10 for each streamer 12. Attaching and then detaching the external retriever modules 10 can be time consuming, increasing the cost of seismic surveys. Additionally, because the typical externally mounted pneumatic retriever modules 10 extend away from the seismic cable 12, they can become fouled on underwater objects and on other cables. In addition, externally mounted retriever modules 10 increase noise within the cable array as the streamers 12 are pulled through the water. For these reasons, there is a need for an improved design for a pneumatic retriever module to a marine seismic streamer cable or other marine tool.
There is further a need to provide a retriever that is an integral part of the cable streamer. Still further, there is a need to provide a retriever which does not need to be attached and detached from the cable line each time the cable is spooled or otherwise handled. Further still, there is a need for a retriever that minimizes acoustic noise generation from the retriever.
The present invention provides an apparatus for retrieving a seismic cable streamer or other marine tool, and a method for using the retriever assembly. The retriever of the present invention defines an in-line retriever, meaning it is placed in series with the cable streamer itself rather than being attached as an external module. Each section of cable includes electrically connected seismic sensors, such as hydrophones.
The novel in-line retriever of the present invention first comprises a cylindrical housing. The housing encloses all of the components required for independent operation of the retrieving device. These components first include a bladder which serves as an inflatable bag. The retriever further comprises a gas canister adapted to hold a quantity of compressed gas such as carbon dioxide or other appropriate non-explosive gas. A gas conduit connects the gas canister with a port in the inflatable bag in order to provide a fluid connection. The in-line retriever further comprises an actuating mechanism which causes pressurized gas to be released through the fluid conduit and into the inflatable bag. The actuating mechanism is pressure sensitive. In this respect, it is triggered by an increase in pressure due to hydrostatic head when the cable loses buoyancy and begins to sink deeper into the body of water.
A pop-open cover is provided on the housing. Inflation of the bag forces the cover on the housing to open, thereby releasing the bag from the housing and allowing further inflation. Complete inflation of the bag allows the cable to buoyantly rise to the surface in accordance with Archimedes principle and other laws of physics.
The housing preferably also provides couplings at either or both ends for physically connecting the retriever to the cable sections. The housing also permits through-passage of required electrical and/or optical transmission apparatus, e.g., hydrophone sensors, wiring, and analog-to-digital converters as may be utilized in marine seismic surveying operations. In this regard, the coupling are compatible with the electrical and optical functions of the cable, permitting the streamer to pass electrical, optical or other necessary signals through the retriever.